Eigenmannia virescens – Glass Knifefish

Eigenmannia virescens – Glass Knifefish

Eigenmannia virescens was first described by Valenciennes in 1836. Originally this species received the name Sternachus virescens, this name is currently seen as a synonym. Other synonyms are: Sternopygus tumifrons and Sternopygus lineatus. The name of the genus Eigenmannia refers to the scientist Eigenmann.

Description

The body of Eigenmannia virescens is green but transparent. This makes it possible to see the internal organs and the spine of the fish. At the lower half of the body, two dark, horizontal lines can be seen. In addition, this species lacks a caudal fin. The shape of the body makes it look similar to a knife.

Differences between the sexes can be seen on the basis of the size of the fish. Males can grow to become twice the size of a female. Males can grow to twice the size of females.

Distribution and Habitat

Eigenmannia virescens has a rather large distribution throughout South-America. It can be found East of the Andes. Here it can be found in waters throughout the northern part of the continent.

Its large distribution has the consequence that it can be found in different biotopes. They occur in both, black and white water. Despite this, the fish mainly inhibits still-deep waters on a substrate of plant debris. Here they like to stay near the soil, which often consists of plant parts.

Diet

These knifefish are carnivorous. Their diet can consist of Tubifex, earthworms, snails, and small fish. Eventually, the fish can be learned to eat frozen or fabricated foods. Some specimens even end up eating flakes or granules.
When looking for food, the Eigenmannia virescens uses weak electrical signals. These signals are used to actively search for prey. They will actively swim through the tank and not lie in an ambush waiting for a victim to swim by. Once the fish have found their prey, they will swim past it backward. This allows the fish to “scan” their prey and know exactly where it is located. During this process, they position themselves so that their head moves towards their prey. Once the food is near their mouth, they will consume it with a quick movement.

The aquarium

In addition to many plants, the aquarium must also contain sufficient hiding places in the form of wood and stones. The fish will appreciate floating plants as they dampen the light. There should be no to a weak current in the water. Make sure you have a strong filter, as the fish cannot withstand pollution and changing water conditions.
Its carnivorous diet and considerable size make it an unsuitable cohabitant for small fish species. Small fish will be seen as food. They will be peaceful towards other larger fish. The fish have complex social relationships with each other. Conspecific members of Eigenmannia virescens can communicate with each other by means of weak electrical pulses. It is striking that the pulses of females are stronger than males. The fish therefore use the pulses not only for detecting objects, but also for mutual communication. Despite this mutual communication, fights can occur. These often cause no injuries.

Breeding

Breeding these fish can be tricky. To encourage the Eigenmannia virescens to spawn, the rainy season can be simulated in an aquarium with soft acid water (DH between 2 and 4). This can be done by pouring cooler water onto the water with a watering can over the course of several hours. The eggs will then be deposited in the morning. This happens in several successive pairings, each time laying a number of eggs. The eggs will be laid between the roots of floating plants. The entire mating takes 3 to 6 hours. A fish can eventually lay 100 to 200 eggs. The eggs can then be removed from the parents. The young fish can then be fed with Artemia nauplii.

Video

Author

R. Eltingh

Copyright images

John de Lange

Resources

Eigenmannia virescens summary page. (n.d.). Retrieved July 15, 2017, from http://www.fishbase.org/summary/12221

Hopkins, C. D. (1974). Electric communication: functions in the social behavior of Eigenmannia virescens. Behaviour, 50(3), 270-304.

Kramer, B., & Otto, B. (1988). Female discharges are more electrifying: spontaneous preference in the electric fish, Eigenmannia (Gymnotiformes, Teleostei). Behavioral Ecology and Sociobiology, 23(1), 55-60.

M. (n.d.). Glass Knife Fish | Eigenmannia virescens. Retrieved July 15, 2017, from http://fish.mongabay.com/species/Eigenmannia_virescens.html

Nanjappa, P., Brand, L., & Lannoo, M. J. (2000). Swimming patterns associated with foraging in phylogenetically and ecologically diverse American weakly electric teleosts (Gymnotiformes). Environmental Biology of Fishes, 58(1), 97-104.